Conventional printed circuit cards and circuit boards comprise a laminated non-conductive dielectric substrate that utilizes drilled and plated through-holes (i.e., PTHs) for communication between opposite sides and/or intermediate layers of the laminated substrate to wiring formed on both sides of the substrate. The plated through-holes are typically formed by applying a metal seed layer on the surface of a laminated non-conductive dielectric substrate prior to deposition of a metal such as copper. The seed layer is employed to ‘catalyze’ the surface of the laminated non-conductive dielectric substrate prior to metal deposition. The use of seed layers is necessary because the laminated substrate is comprised of non-conductive dielectric materials.
Among one of the more widely employed procedures for catalyzing the laminated non-conductive dielectric substrate is to treat the laminated substrate with a solution containing a stannous chloride sensitizer solution and a palladium chloride activator.
For instance, one method for catalyzing a dielectric substrate is exemplified by U.S. Pat. No. 3,011,920 to Shipley, Jr. which includes sensitizing the substrate by first treating the substrate with a solution of a colloidal metal, accelerating the treatment with a selective solvent to remove protective colloids from the sensitized dielectric substrate and then electrolessly depositing a metal coating on the sensitized substrate; for example, with copper from a solution of a copper salt and a reducing agent.
U.S. Pat. No. 4,008,343 to Cohen, et al. and U.S. Pat. No. 3,562,038 to Shipley, Jr., et al. also relate to methods for catalytically activating a surface of a non-conductive dielectric substrate with solutions containing Pd and Sn salts.
There have also been suggestions in the patent literature of treating substrates with certain surfactants. In particular, U.S. Pat. No. 4,301,190 to Feldstein suggests pretreatment of a substrate with an “adsorption modifier” to enhance the attachment to the substrate of a non-noble metal catalyst. Certain surfactants, hydride oxide sols and certain complexing agents are suggested as “adsorption modifiers”. In particular, fluorocarbon surfactants and silicon-bearing surfactants are disclosed in Feldstein.
U.S. Pat. No. 3,563,784 to Innes, et al. suggests a method of pretreating non-conductors for plating including a step of treating the surface with certain monofunctional surfactants, rinsing, and then activating either by a two-step stannous chloride-palladium chloride treatment or a one-step acid, tin-palladium hydrosol treatment. The monofunctional surfactants disclosed in Innes, et al. include tallow trimethylammonium chloride, nonylphenoxypoly(ethyleneoxy) ethanol and acid phosphates.
U.S. Pat. No. 3,684,572 to Taylor relates to a method of plating non-conductors including steps of treating the surface of the non-conductors with certain quaternary amine monofunctional or single charged surfactants after etching and before catalyzing the surface.
U.S. Pat. No. 3,573,973 to Drotar, et al. relates to a process for plating a non-conductive substrate which includes a step of rinsing the substrate with certain detergents before sensitizing with stannous chloride and activating with palladium chloride.
U.S. Pat. No. 3,515,649 to Hepfer; U.S. Pat. No. 3,877,981 to Arnold; and U.S. Pat. No. 3,930,072 to Wilks are of interest in that the references disclose the use of surfactants in plating processes in steps prior to the deposition of a catalyst. Specifically, Hepfer discloses fluorocarbon surfactants; Arnold discloses low molecular weight alcohols such as propanol as the surfactant; and Wilks discloses alkylaryl polyether alcohols, sulfonates, sulfates or sorbitan derivatives as surfactants.
U.S. Pat. No. 3,421,922 to Wilson describes the application of a cationic film forming resin, specifically, melamine-formaldehyde resins, polyalkylene-amines, alkylated-methylol-melamines, triazine-formaldehyde and urea formaldehyde resins, onto surfaces of the substrate which are to be subsequently plated.
In co-assigned U.S. Pat. No. 4,478,883 to Bupp, et al., a method for conditioning surfaces of dielectric substrates for the electroless plating is disclosed in which a copolymer of a non-reactive backbone chain of polyacrylamide to which are attached short chains of a tetra-alkyl-ammonium compound, with multiple charge functionality dissolved in a diluted inorganic acid, such as hydrochloric acid, is applied to the substrate surface prior to activation.
Co-assigned U.S. Pat. No. 4,448,804 to Amelio, et al. provides a multistep process for the electroless plating of Cu onto a non-conductive surface. The seed bath includes palladium chloride, stannous chloride and HCl.
Co-assigned U.S. Pat. No. 4,554,182 to Bupp, et al. provides a method for the electroless plating of a metal such as copper onto non-conductive substrate surfaces.
This prior art method comprises bringing the surfaces of the substrate into contact with an aqueous composition containing H2SO4 and a multifunctional cationic copolymer containing at least two available cationic moieties and then activating the surfaces by treating them with a colloidal solution containing palladium chloride, stannous chloride and HCl.
In the '804 patent and the '182 patent mentioned above, the colloidal seed formulations disclosed therein contain a fluorocarbon surfactant such as perfluorooctane sulfonic acid (PFOS). The fluorocarbon surfactants are employed as wetting agents in such seed formulations. Initially, PFOS and other like fluorocarbon surfactants were chosen because of their highly stable structure, which helps in reducing the molecular breakdown of the surfactant in harsh chemical environments. However, it is for a similar reason that some perfluorinated compounds are being targeted for regulatory legislations. Recent EPA actions have dictated the use of PFOS and other like perfluorinated compounds be banned by the end of 2002 due to the bioaccumulative and toxicological properties of such compounds.
Consequently, because the nature of the seeding process for through-hole metallization requires good wetting ability to homogeneously catalyze plating in high-aspect ratio vias, an alternative surfactant is needed which is environmentally friendly. Moreover, a new surfactant for a colloidal seed formulation is needed, which does not negatively affect seed deposition, or require a significant amount of additional bath monitoring by personnel running the metallization process.